<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>sb</title>
    <script type="text/javascript">
    var a;
    a = 21;
    if(a%2 == 0)
    {
        document.write("a是偶数");
    }
    else
    {
        document.write("a是奇数");
    }
    </script>
</head>
<body>
    
</body>
</html>